Method of making malleable iron



Nov. 3, 1931. vALENTlNE 1,830,630

METHOD OF MAKING MALLEABLE IRON Filed July 16, 1928 Figl.

735. 629700 mow .xr/m sommflmo TEMPERATURE H UURS m F 5 E 70 g 600 m 3, 5 95 E a "g' 7.7600 601270 56400 E 25% HOURS HauRs I000 I000 800 F g.5. m 900 E 00 E 00 F g4 E 5 HOURS Inventor:

Tr-vin Ryalenttne,

Hts Attorney.

HOURS Patented Nov. 3, 1931 IBVINGB. VALENTINE, OF ERIE, PENNSYLVANIA, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF YORK METHOD or MAKING WEABLE molt Application filed July 16, 1928. SeriaI No. 293,276.

. This application is a continuation in art of my prior application, Serial No, 233, 23,

filed November 16, 1927.

a The present invention relates to a method of making malleable iron from white cast iron, the latter consisting of cementite, which is a form of iron carbide, and iron. It has hitherto been customary, in the manufacture of malleable iron to heat gradually White cast iron, made from a chemically determined mixture of selected pig iron, steel and, sprue, to a temperature of about 880 (1., to maintain this temperature for a period of about 60 hours and then gradually reduce it in about an 80 hour period to a temperature of about 500 C., the total period of time employed in such a process being about 180 hours. 4

According to my invention, I provide an improved method for converting white cast iron into malleable iron whereby the annealing period ma be materially shortened.

The novel eatures whichl believe to be characteristic of the present invention are set forth with particularity in the appended claims. The invention itself, however, will best be understood from reference to the following specification when considered in connect-ion with the accompanying drawings in which Figs. 1 to 5 inclusive illustrate the relation between the temperature and the time employed in carrying the invention into effect.

In carrying out my invention, white iron castings, such as usually employed in the process of making malleable iron, are heated to a temperature of about 1000 C. as indicated in the difierent figures of the drawings. This temperature is maintained for a period of about four hours or long enough to cause the cementite or carbon in the iron to go into solid solution. After this has been elfected the iron is ready for the precipitation of graphite (temper carbon). This latter step is carried out by cooling the iron from the 1000 C. temperature to the temperature of a second heating zone which has for best results an upper limit of about 750 .of time which may va until substantially complete graphitization has taken place. The castings are main tained in the second heating zone for a period from about 4 to about 36 hours after which they are cooled m air or quenched in any desired manner. If the temperature in the second heatingzone is maintained substantially constant for a short period of time and then changed abruptl a relatively small amount, the time require to properly anneal the iron will be quite short, whereas if the temperature is maintained substantially constant throughout the second zone, the time required to annealithe castings will be appreclahly lengthene lron bars heated to about 1000 C. for four hours and then heated to a substantially constant temperature of about 730 in the second heating zone are completely annealed at the end of a period of about 22 hours, as indicated in Fig. 1, i. e., bars tested after such a heat treatment have a tensile strength of about 55,100 pounds per square inch and an elongation of about 14%. (lontinuation of the heat treatment in the 700 C. to 750 C. zone has the effect of reducing the tensile strength and increasing the elongation. For example, at the end of 28 hours a bar heated at a temperature of 720 in the second heating zone had a tensile strength of 48,300 pounds per square inch and an elongation of 16.5%. These latter figures compare favorably with ordinary malleable iron in which the tensile strength averages about 50,000 pounds per square inch while the elongation averages about 10%.

Although heating the casting at a temperature of about 1000 C. for about 4 hours in the first heating zone and then annealing it at a substantially constant temperature in the second heating zone produces malleable iron in a comparatively short period of time, this time may be still further reduced if, instead of holding the iron at a given constant temperature in the '7 00 to 750 C. zone, the temperature is reduced from the 1000 G. temperature in the initial heating zone to a temperature of say 735 (3., maintained at the latter level for about four hours then dropped to 725 (3., kept at the latter .level for about four hours, then dropped to 715 C. and maintained at that temperature similar manner to that indicated in' Fig. 2

and for a period of about 11 to about 14 hours, as illustrated in Figs. 3, 4 and 5, have shown similar characteristics. The duration of the difierent heating periods in the second .zone instead of being 4 hours each ma be 2 hours each, as indicated in Figs. 4 and 5, and from 2% to 4hours, as indicated in Fig. 3.

In Figs. 2 and 3 of the drawings, I have indicated the use of small abrupt downward changes of temperature in the second heating zone. The changes in the second zone, however, may be made either upwardly or downwardly as desired, provided the temperature is kept within the llmits of the zone. As illustrated in Fig. 4, for example, the temperature of the casting was dropped from 1000 to about 715 C. and then raised to 725 and 735 C. at the end of 2 and 4 hours respectively. Under these conditions .less than 5 hours was required in the second heating zone to completely malleabilize the iron.

If the temperature of the casting is dropped from 1000 C. in the first zone to a temperature lower than 700 C. and later raised from this latter temperature to a temperature of say 715 C. the annealing period will be lengthened beyond that indicated in Figs. 2, 3 and 4. However, when the temperature is dropped from 1000 C. to a temperature lower than 7 00 C., the casting may still be properly annealed in a very shortperiod of time if, as shown in Fig. 5, its temperature is raised abruptly from 7 this lowentemperature to a temperature of about 800 C, to 850 C. and then abruptly dropped to a temperature in a zone between 750 C. and 700 C. and given one or more relatively small abrupt changes in temperature in this zone. indicated in Fig. 5, a casting treated in this manner and cooled in air was completely annealed about 10 hours after it had reached a temperature of about 1000 in the first heating zone.

Although the castings are heated initially for a four hour period at a temperature of- 1000 0., this temperature is subject to some variation. It will be found undesirable, however, to heat the casting initially to a temperature lower than 940 C. or higher than 1100 C. If the casting is heated to 940 (3., at least eigth hours will be re uired to effect solution of carbon or cementlte which will lengthen the total annealing time. If the castin is heated to 1100 C. for four hours there 1s tendency to burn out more carbon at the rim of the casting than is desirable, with the result that when the castingis malleabilized there is a steely looking rim at the surface of the casting. This of course would not take place if the castings were protected from oxi ation by the use of a vacuum furnace or salt heating-bath. The average electric furnace, how ver, is not sufliciently airtight to overcome this difiiculty. At a temperature of about 1100 C. the iron is close to the softening point. The solution effect of the cementite increases very rapidly with increased temperature, the solution period being lengthened or shortened with temperatures lower than or higher than 1000 C., respectively. I

If the temperature in the second zone is as low as 675 C. or as high as 755 C. there is a tendency to roduce a structure which is somewhat perlitic, and although it is possible to anneal castings at temperatures of about 675 C. or 7 55 C. it would take from 36 to 48 hours to break up the perlite which has formed. The best temperature range in the second heatin zone will, therefore, lie between these limlts. If the temperature is maintained substantially constant in the second heating zone, a temperature of about 725 C. will be'found to give satisfactory results. However, equally good results may be obtained with a shorter heating period if'the temperature in the second heating zone instead of being maintained constant is stepped up or down once and preferably I twice. Although I have indicated a change intemperature at each step of about 10 C. the change at each step may be greater or less if desired. It is desirable however that the temperature should be maintained well within the limits of 700 and 750 C.

The change in temperature from the first heating zone to the second heating zone may be effected by cooling in air or in any suitable manner until the casting temperature is reduced to the temperature of the second zone. If reduced to 715 or 735 C. the casting, in order to be annealed in the shortest period of time, should be maintained at that temperature for about two to four hours after which the temperature is given one or more small abrupt changes. If 715 C. is the initial temperature in the second zone, it should be raised in successive steps to about 725 and 735 C. respectively, and correspondingly reduced if 735 C. is the initial temperature, the duration of each step varying from about two to about four hours.

able iron may be still further reduced if the process is carried out in the absence of air. For example, if the iron is heated for four hours at 1000 C. in a bath of molten salt,

such as barium chloride, and then transferred to a furnace at 725 C. or to a molten salt bath having the latter temperature, the total heating period necessary to produce the malleable iron will be about sixteen hours, twelve hours being required at the lower temperature and four hours at the higher temperature. The use of a salt bath not only slightly reduces the time necessary to malleabilize the iron but prevents the formation of excessive scale. Of course if the temperature in the second zone were stepped down as in Figs. 2 and 3. or up as in Figs. 4 and 5, the time required to produce the malleable iron would be still further reduced.

If desired, malleable iron produced according to my improved process may be galvanized without undergoing any aging or heat treatment other than that given in my process of making the malleable iron. Tests on castings produced by my process and galvanized, have shown a tensile strength of about 30.120 pounds per square inch and entire freedom from brittleness even when subject to cold treatment below 0 C.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of making malleable iron which comprises heatingwhite cast iron for a period of time and at a temperature sufliciently high to cause the cementite in the white iron to go into solid solution, cooling the cast iron to a temperature not lower than 675 0., again heating it at a lower temperature than Said first mentioned temperature but within a range varying from 675 C. to 7 50 C. and maintaining the temperature in said range for a period of time sufficient to cause substantially complete graphitization of the cast iron, the total time required to heat the cast iron at said first mentioned temperature and in said range being not materially greater than 36 hours.

2. The method of making malleable iron which comprises heating white cast iron at a temperature of approximately 1000 C. for

about four hours, then reducing 'the temperature of the iron to a temperature between 675 C. and 750 C. and maintaining said temperature for a period of time varying from .eight to thirty-six hours and then cooling the castin 3. Tdie method of making malleable iron which comprises heating white cast iron for a period of time and at a temperature sufliciently high to cause the cementite to go into solid solution and then heating the cast iron at a materially lower temperature but within a temperature range above 675 C. and lower than 750 0., the temperature in said range being maintained substantially constant for a few hours, and then reduced slightly and again maintained substantially constant for a few hours.

4. The method of making malleable iron which comprises heating white cast iron for a period of time and at a temperature sufficiently high to cause the cementite to go into solid solution and later heating the cast iron at a materially lower temperature but within a temperature range above 675 C. and lower than 750 0., the temperaturein said range being maintained substantially constant for a few hours and then changed abruptly a relatively small amount and again maintained substantially constant for a fewhours.

5. The method-of makin malleable iron which comprises heating w ite cast iron at a temperature of approximately 1000 C. for about four hours then heating the casting in successive stages, the temperature of each stage being substantially constant but difierent from the temperature in the other stages, the temperature in the successive stages being within the limits of 675 C. to 7 50 C.

6. The method of finishing the malleabilizing of white cast iron which has been heated to a temperature sufiiciently high to cause the cementite to go irito solid solution, which I comprises heating the casting for a few hours at a temperature within the range from 700 C. to 750 C. and abruptly changing the temperature of the casting while maintaining the temperature within the said range.

7. The method of making malleable cast 5 iron which comprises the initial step of heatingwhite cast iron to a temperature of about 1000 C. for a few hours, the step of heating the casting in a finishing tem erature zone having an upper limit of a out 750 C. and a lower limit of about 700 (1., and an intermediate series of steps which comprise cooling the casting from the initial temperature of 1000 C. to a temperature lower than the temperature of the finishing zone,

raising the temperature abruptly above the temperature of the finishing zone and reducing the temperature of the casting to the temperature of the finishing zone.

8. The method of making malleable cast iron which comprises heating white cast iron at an elevated temperature and for a period of time sufficient to cause the cementite to go into solid solution then cooling the casting to atemperature' zone having an upper limit of about 735 C. and a lower limit of about 700 0., heating the castingin saidtemperature zone for more than six hours,'and varying,

at substantially equal intervals, the temperature of the casting in the zone.

9. The method of making malleable cast iron which comprises heating white cast iron at an elevated temperature and for a period of time suflicient to cause the cementite to go into solid solution then cooling the casting to a temperature zone having an upper limit of about 735 C. and a lower limit of about 7 00 C. and heating the casting in said temperature zone until graphitization of the cast iron is substantially complete, the total time required at said elevated temperature and in said temperature zone being not materially greater than 36 hours.

10. The method of making malleable cast iron which comprises heating white cast iron to a temperature of about 1000 C. for a few hours then cooling the casting to a temperature zone having an upper limit of about 735 C. and a lower limit of about 700 C. and heating the casting in said temperature zone for a period of time varying from about six hours to about thirty-five hours.

11. The method of making malleable iron which comprises heating white cast iron at a temperature sufiiciently high to cause the cementite to go into solid solution and later heating the casting in different successive temperature stages, each stage having a duration of about'four hours, the temperature in each of said different stages being within the limits of about 700 C. to about 750 C.

12. The method of making malleable iron which comprises heatingwhite cast iron at a temperature sufliciently high to cause the comentite to go into solidijsol'ution and later heating the casting in difierent successive temperature stages, each stage having a duration of a few hours, the temperature in each stage being substantially constant and ){Vithig the limits of about 700 C. to about Iii witness whereof, I have hereunto set my hand this 12th day of July 1928.

IRVING R. VALENTINE. 

